Bug fix for ensuring location and category names are unique for common parent (#4361)

* Update Meta class for StockLocation and PartCategory

* Migration files

* Add extra unique requirements to  InvenTreeTree model

- unique_together does not work as expected with null values

InvenTree/part/models.py

@@ -66,6 +66,11 @@ class PartCategory(MetadataMixin, InvenTreeTree):
         default_keywords: Default keywords for parts created in this category
     """
 
+    class Meta:
+        """Metaclass defines extra model properties"""
+        verbose_name = _("Part Category")
+        verbose_name_plural = _("Part Categories")
+
     def delete_recursive(self, *args, **kwargs):
         """This function handles the recursive deletion of subcategories depending on kwargs contents"""
         delete_parts = kwargs.get('delete_parts', False)
@@ -154,11 +159,6 @@ class PartCategory(MetadataMixin, InvenTreeTree):
                   "are already assigned to it!"))
         super().clean()
 
-    class Meta:
-        """Metaclass defines extra model properties"""
-        verbose_name = _("Part Category")
-        verbose_name_plural = _("Part Categories")
-
     def get_parts(self, cascade=True) -> set[Part]:
         """Return a queryset for all parts under this category.
 
@@ -747,7 +747,7 @@ class Part(InvenTreeBarcodeMixin, MetadataMixin, MPTTModel):
             return helpers.getBlankThumbnail()
 
     def validate_unique(self, exclude=None):
-        """Validate that a part is 'unique'.
+        """Validate that this Part instance is 'unique'.
 
         Uniqueness is checked across the following (case insensitive) fields:
         - Name